This invention relates to high frequency amplifier circuits. More particularly, it is concerned with high voltage, high frequency amplifier circuits employing power transistors.
Transistor operating voltages are limited by inter-region breakdown voltages, in the case of bipolar transistors BV.sub.CES (collector-to-emitter breakdown voltage) and in the case of field effect transistors BV.sub.DSS (drain-to-source breakdown voltage). When a field effect transistor is used as the active component in a class C amplifier and the output impedance of the transistor is properly matched, the transistor drain voltage level normally swings from the drain-source saturation voltage to approximately twice the supply voltage. Similar voltages occur in class C amplifiers using bipolar transistors. Under certain conditions voltage standing waves with peak amplitudes greater than twice the power supply voltage are possible. Therefore, supply voltage levels somewhat lower than one-half the maximum breakdown voltage value are normally employed in amplifiers of this type.
Ideally, in order to achieve high output power transistors having high breakdown voltage capabilities would be employed biased with high supply voltages. High breakdown voltage and high frequency operation, however, are technically conflicting requirements in semiconductor device design. Because of the thin interaction regions within devices necessary in order to achieve high operating frequencies, high frequency devices have inherently low breakdown voltages.
Therefore, in order to increase output power at high frequencies, parallel active devices in a single amplifier stage or parallel power amplifiers are commonly utilized. The drive power is divided and applied to the individual amplifying devices, and the output powers are then combined to achieve the desired high power level. These arrangements provide a higher power amplifier output but each active device operates at the same dc supply voltage and the amplifier draws a high dc current level depending on the number of parallel power units.